Scale inhibitors are applied in production wells to prevent precipitation of water-borne deposits, such as barium sulfate, calcium carbonate, and calcium sulfate. Such precipitates can hinder fluid production by blocking the flow paths either inside the formation or at the perforations. Scale deposition also can appear in the tubing, slowing fluid production and damaging downhole equipment.
One common method of applying a scale inhibitor is the "squeeze process." The steps involved in the squeeze process include:
1. injecting the aqueous scale inhibitor solution (often a low percent inhibitor concentration),
2. injecting an overflush brine solution to push the scale inhibitor several feet away from the wellbore,
3. shutting-in the well for about a day to allow maximum retention of the inhibitor on the rock surfaces, and
4. putting the well back on normal production.
The produced water then slowly leaches the retained scale inhibitor from the formation. Ideally this places a low, but still effective, concentration (typically a low mg/l) of the scale inhibitor into the produced water to prevent scale deposition for many weeks or even months.
Measurement of the concentration of scale inhibitor after such a squeeze treatment often shows:
1. a short period of no inhibitor while the overflush is produced back;
2. a rapid increase of inhibitor concentration for a short time, representing material not retained well in the formation; and
3. a gradual decline of inhibitor concentration.
This continues until the inhibitor concentration finally falls to the minimum effective concentration. The "squeeze lifetime" is the length of time to this point. Then the squeeze must be repeated.
The squeeze process is chemically inefficient. The inhibitor concentration is higher than necessary, particularly in the early stages, and a significant amount of the inhibitor often remains adsorbed inside the formation after the squeeze. Usually two-thirds of the injected scale inhibitor is wasted.
An ideal scale inhibitor return curve for scale inhibitor concentration shows that, once the overflush is brought back, the inhibitor desorbs into the produced water such that the chemical concentration is just above the minimum effective level. This continues until all squeezed inhibitor is released from the formation. Such an ideal process would have a chemical efficiency of almost 100%, or equivalently, a longer squeeze life with the same amount of scale inhibitor.